A transmission device that is configured as an 8-speed multi-stage transmission having frictionally engaged shifting elements, such as lamellar clutches and multi-disk brakes, is known from the document DE 10 2005 002 337 A1. When a shifting request is made for a gear change in the transmission device, at least one frictionally engaged shifting element, which is connected in the flow of power of the transmission device in order to obtain the actual gear ratio currently selected in the transmission device, must be disconnected from the flow of power of the transmission device, while at least one additional frictionally engaged shifting element, which is disconnected from the flow of power of the transmission device while obtaining the actual gear ratio currently selected in the transmission device, must be connected in order to obtain the requested actual gear ratio in the flow of power of the transmission device.
In so doing, the torque that is applied via the shift element connected in the flow of power in order to obtain the current actual gear ratio of the transmission device, is more or less transferred from the frictional shift element that must be connected in order to obtain the requested target transmission ratios with an increased shift time, while the torque that is applied via the shift element that must be disconnected decreases.
Frictionally engaged shifting elements in a disengaged operating state disadvantageously causes drag torques, which affect the overall efficiency of an automatic transmission to an undesirable extent.
For this reason, transmission devices as disclosed in what is regarded as the closes prior art, DE 10 2008 000 429 A1, are increasingly designed having not only frictional shifting elements, but also with form-fitting shifting elements in the region in which no drag torques occur that affect the overall efficiency of a transmission device.
It should also be noted, however, that form-fitting shifting elements can only be shifted from a disengaged operating state, in which no torque can be applied via the form-fitting shifting elements, into its engaged operating state, in which the entire applied torque can be applied via the form-fitting shifting elements when the form-fitting shifting elements are near their synchronization point. In addition, form-fitting shifting elements that are connected in the flow of power of a transmission device with low shift forces can only be disconnected from the flow of power when they are near their load-free operating state. In contrast to frictionally engaged shifting elements, additional structural bodies are needed, both for the synchronization of form-fitting shifting elements and for transferring form-fitting shifting elements into their load-free state, in order to be able to carry out shifts in transmission devices, in which at least one form-fitting shifting element is involved, within the desired shift times.
In order to be able to carry out a requested gear change from an actual gear ratio in the direction of a target gear ratio to the desired extent with a high shift quality or a high degree of shift comfort using the above described transmission devices, the implementation of which requires that a form-fitting shifting element be transferred from its engaged operating state into its disengaged operating state and at the same time, a frictionally engaged shifting element be transferred from its disengaged operating state into its engaged operating state, the form-fitting shifting element that is to be disconnected must be disengaged at the right point in time, that is, within an operating state window, during which the torque applied to the form-fitting shifting element is substantially at least approximately equal to zero.
If the form-fitting shifting element is disengaged by a premature or late actuation of the form-fitting shifting element, for example, there is a possibility that at least a part of the torque applied via the transmission device will be applied to the form-fitting shifting element at a predefined target moment of disengagement and the disengagement of the form-fitting shifting element in a vehicle drive train equipped with the transmission device will produce an unwanted drive train reaction as a result of the abrupt reduction in the torque applied to the form-fitting shifting element, which the driver of a vehicle that implements the vehicle drive train will perceive as an unpleasantly noticeable jolt.
Moreover, in the event of a disengagement or release attempt that is too late, wherein the transmission capability of the frictionally engaged shifting element that is to be connected is already too high, there is the possibility that the form-fitting shifting element can only be transferred into its disengaged operating state by applying undesirably high shift forces, which are caused due to high frictional forces between the inter-engaging halves of the shifting element created by tensions in the region of the transmission device.